## 200. Number of Islands

Given an `m x n` 2d `grid` map of `'1'`s (land) and `'0'`s (water), return the number of islands.

An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

```Input: grid = [
["1","1","1","1","0"],
["1","1","0","1","0"],
["1","1","0","0","0"],
["0","0","0","0","0"]
]
Output: 1
```

Example 2:

```Input: grid = [
["1","1","0","0","0"],
["1","1","0","0","0"],
["0","0","1","0","0"],
["0","0","0","1","1"]
]
Output: 3
```

Constraints:

• `m == grid.length`
• `n == grid[i].length`
• `1 <= m, n <= 300`
• `grid[i][j]` is `'0'` or `'1'`.

## Rust Solution

``````struct Solution;

struct UnionFind {
parent: Vec<usize>,
n: usize,
}

impl UnionFind {
fn new(n: usize) -> Self {
let parent = (0..n).collect();
UnionFind { parent, n }
}

fn find(&mut self, i: usize) -> usize {
let j = self.parent[i];
if i == j {
i
} else {
let k = self.find(j);
self.parent[i] = k;
k
}
}

fn union(&mut self, mut i: usize, mut j: usize) {
i = self.find(i);
j = self.find(j);
if i != j {
self.parent[i] = j;
self.n -= 1;
}
}
}

impl Solution {
fn num_islands(grid: Vec<Vec<char>>) -> i32 {
let n = grid.len();
if n == 0 {
return 0;
}
let m = grid[0].len();
let mut uf = UnionFind::new(n * m + 1);
for i in 0..n {
for j in 0..m {
let land = grid[i][j];
if land == '1' {
if j > 0 && grid[i][j - 1] == '1' {
uf.union(i * m + j, i * m + j - 1);
}
if i > 0 && grid[i - 1][j] == '1' {
uf.union(i * m + j, (i - 1) * m + j);
}
} else {
uf.union(i * m + j, n * m);
}
}
}
(uf.n - 1) as i32
}
}

#[test]
fn test() {
let grid: Vec<Vec<char>> = vec_vec_char![
['1', '1', '1', '1', '0'],
['1', '1', '0', '1', '0'],
['1', '1', '0', '0', '0'],
['0', '0', '0', '0', '0']
];
assert_eq!(Solution::num_islands(grid), 1);
let grid: Vec<Vec<char>> = vec_vec_char![
['1', '1', '0', '0', '0'],
['1', '1', '0', '0', '0'],
['0', '0', '1', '0', '0'],
['0', '0', '0', '1', '1']
];
assert_eq!(Solution::num_islands(grid), 3);
}
``````

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